Wastewater treatment by catalytic oxidation

ABSTRACT

Waste water chemical oxygen demand is reduced from a waste water by passing the water in the presence of an oxidizing agent through an adsorbent porous solid substrate, preferably zeolite A, zeolite X, zeolite Y, ZSM-5, erionite, chabasite or activated carbon which has been partially ion exchanged with a water insoluble metal compound, preferably copper (Cu), that facilitates oxidation of the components in the waste water that increase its chemical oxygen demand, such as sulfide, thiosulfate, sulfite, mercaptan, or disulfide. The preferred oxidizing agent is air.

FIELD OF THE INVENTION

This invention is directed to reducing the chemical oxygen demand ofwaste water by oxidizing various compounds including, for example,sulfides, sulfites, thiosulfates, mercaptans, and disulfides using anadsorbent substrate treated with a water insoluble compound along with asource of oxygen.

BACKGROUND OF THE INVENTION

Waste water streams from industrial plants, including petroleumrefineries, chemical plants, pulp and paper plants, mining operationsand food processing plants, can contain toxic substances such ascyanides, sulfides, sulfites, thiosulfates, mercaptans, and disulfidesthat tend to increase the chemical oxygen demand (COD) of the wastewater streams. Examples of these waste water streams in petroleumrefineries include sourwater, sourwater stripper bottoms, and spentcaustics. The Environmental Protection Agency (EPA) and various localagencies have placed limits on the allowable levels of these toxins inindustrial waste water effluent streams.

The conventional methods for wastewater treatment include incineration,biological oxidation, and chemical oxidation using H₂ O₂, C1₂, NaOC1,C1O₂, and KMnO₄. The concentration of the toxin in the waste water maybe too low to treat economically using conventional means.

U.S. Pat. No. 5,112,494 to Yan teaches a method of removing cyanide fromcyanide-containing waste water utilizing a water insoluble metalcompound deposited onto a porous adosrbent substrate, the entiredisclosure of which is incorporated by reference herein. The patent alsoteaches that a process using a water insoluble metal compound depositedonto a porous adsorbent substrate is effective in reducing the chemicaloxygen demand of cyanide-containing waste water by oxidizing theoxidizable components of the water.

It is well known that sulfides in waste water, including sourwaterstripper bottoms or foul water, can be oxidized using air to reduce thechemical oxygen demand of the waste water. These air oxidizationprocesses are commonly practiced in petroleum refineries. In these airoxidation processes, the sulfides are oxidized to thiosulfate as isshown in the following representation:

    2S.sup..sup.50 +20.sub.2 +H.sub.2 O→S.sub.2 O.sub.3.sup.═ +20H.sup.-

S₂ O₃.sup.═ +20₂ +H₂ O→2SO₄.sup.═ +H⁼

As noted in Abegg et. al. ("A Plant for Oxidation of Sulfide ContainingRefinery Waste by Air", Ardol Kohle Erdgas Petrochemie, Sept. 1962), thereaction rate of sulfide to thiosulfate in the presence of air, asrepresented by equation (1) above, is relatively rapid. Unfortunately,the reaction rate of thiosulfate to sulfate as represented by equation(2) above, is extremely low, so that in an air oxidation process, mostof the sulfides are converted to thiosulfate. A second, more severeprocess is required to oxidize the thiosulfate to sulfide. Based onAbegg's data, Beychok ("Aqueous Wastes from Petroleum and PetrochemicalPlants," page 210 John Wiley, 1967) observed that, "To oxidize 34% ofthe sulfides to sulfates requires a tenfold increase in tower volumecompared with units that oxidize the sulfides to thiosulfate." Thus, acatalyst is required to convert sulfides and thiosulfate efficitnely tosulfate.

Copper is an effective catalyst for oxidation of sulfides andthiosulfate. Beychok also observed that by use of homogeneous CuC1₂catalyst, sulfides can be converted completely to sulfates. Continuousaddition of a homogeneous catalyst to the treatment system isundesirable because of the chemical and operating costs, and mostimportantly, pollution of the treated water by copper.

In developing water treatment processes, particular concern is directedto processes which do not leave residues in the treated stream. Residuescan cause additional disposal problems. Materials consumption and costis also an important factor; thus, it is important to avoid processeswhich require replenishing the supply of costly catalyst and reagent.

SUMMARY OF THE INVENTION

A treatment method has now been discovered for waste water in which manytoxins, including sulfides, sulfites, thiosulfates, mercaptans, anddisulfides, that increase waste water chemical oxygen demand (COD) arefound. The process catalytically oxidizes the toxins using a source ofoxygen and an adsorbent substrate treated with a water insolublecompound.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified schematic diagram of the process to oxidizethiosulfates in accordance with the invention.

FIG. 1B is a simplified schematic diagram of the process to oxidizesulfides in accordance with the invention.

FIG. 2 is a simplified schematic diagram of a generalized typicaloxidizing unit for waste water COD reduction.

DETAILED DESCRIPTION OF THE INVENTION

The invention is a process for oxidizing COD causing toxins, includingsulfides, sulfites, thiosulfate, mercaptans, and disulfides from wastewater through an adsorbent substrate treated with a water insolublecompound in the presence of a source of oxygen. The process iseconomically advantageous because it employs a long lasting adsorbentwhich effectively oxidizes the toxins, but does not require continuousreplacement of the active adsorbent component and permits a singlecatalytic process to replace several other processes.

Waste water chemical oxygen demand (COD) can be reduced in the method ofthe present invention by feeding a waste water containing a reducingcompound selected from sulfide, thiosulfate, sulfite, mercaptan, and/ordisulfide along with a source of oxygen into a reaction zone containinga particular porous solid substrate having a water insoluble metalcompound or water insoluble metal compounds deposited thereon. Then thewaste water and the source of oxygen are contacted with the substratewhich catalytically oxidizes the reducing compound, and the waste wateris discharged from the reaction zone whereby the waste water has asubstantially lower concentration of the reducing compound. Theoxidation may be carried out under mild conditions which makes thepresent method easily incorporated into current refinery processes andeasily retrofitted into existing refinery treating systems.

An object of the invention is to effectively and inexpensively reducethe COD of waste water containing reducing compounds, such as sulfides,sulfites, thiosulfate, mercantans, and disulfides. Another object of theinvention is to provide a waste water treating process that consumes nochemicals that lead to additional waste disposal problems. A furtherobjective of this invention is to provide a process that can bepracticed by constructing a new processing unit or modifying an existingprocessing unit.

A feature of the invention is the reduction in waste water COD caused bysulfides, sulfites, thiosulfate, mercaptans, and disulfides by oxidizingthe toxins using a source of oxygen over a water insoluble metalcompound deposited on a porous adsorbent substrate.

An advantage of the invention is the reduced cost and improvedefficiency in the reduction of waste water COD caused by sulfides,sulfites, thiosulfate, mercaptans, and disulfides by employing a porousadsorbent substrate treated with a water insoluble metal compound whichcatalyzes oxidation of the toxins.

A further advantage of this process is that the above catalysis may beaccomplished in one process step.

A further advantage of this process is that the water insoluble metalcompound is not leached from the porous adsorbent substrate andtherefore does not create additional processing requirements.

A further advantage is that the process of this invention is effectiveto reduce the COD of cyanide-free waste water containing other reducingcompounds, such as sulfides, sulfites, thiosulfates, mercaptans, and/ordisulfides.

A further advantage of this invention is that this process can utilizeas the substrate fresh and spent commercial hydrotreating catalysts,e.g. CoMo/A1₂ O₃, NiMo/A1₂ O₃, NiW/A1₂ O₃, or Mo/A1₂ O₃, hydrocrackingcatalysts, e.g. CoMo/A1₂ O₃, NiMo/A1₂ O₃, NiW/A1₂ O₃, Mo/A1₂ O₃,zeolites, or SiO₂ /A1₂ O₃, reforming catalysts, e.g. Pt/A1₂ O₃ orPt-Re/A1₂ O₃, or hydrogenation catalysts, e.g. Pd/C, Ni/Kieselguhr,Pt/C, or Pt/A1₂ O₃. Use of spent catalyst in this process isadvantageous due to cost savings due to catalyst reuse and reduction incatalyst disposal requirements for spent catalyst. Careful selection ofthe specific commercial catalyst is required to avoid problems with thewaste water leaching the metal from the catalyst. The preferredcommercial catalyst for this process is spent hydrotreating catalyst,e.g. CoMo/A1₂ O₃.

Adsorbent substrates which are useful in the method of this inventioninclude porous, high surface area solids. A variety of porous solids canbe employed for purposes of the invention. Non-limiting examples ofporous solids for use herein include activated carbon, inorganicion-exchange materials, polymeric resins (both gel and macro-reticuloustypes), titania, and zirconia.

Specific examples of the inorganic ion exchange materials include boththe naturally occurring materials such as the mineral zeolites includingmordenite, clinoptilolite, erionite, sepiolite, clays and syntheticmaterial, which include A1₂ O₃ SiO₂, SiO₂ --A1₂ O₃, synthetic zeolitessuch as zeolite A, zeolite X, zeolite Y, ZSM-5 and mordenite.

Non-limiting examples of the water insoluble metal compounds which maybe deposited on the porous substrate for use herein are those waterinsoluble compounds which contain a metal having oxidation properties,examples of which include Cu, Co, Ni, Fe, Ag, Cr, Mo, Bi, Hg, Pd, Pt,and Mn and mixtures thereof in the form of metal, sulfide, and oxide.The most important consideration in choosing the chemical state of themetal is its solubility in waste water or its leachability by the wastewater. The metals selected should be very low in solubility in the wastewater and in leachability by the waste water. The desired solubility inwater is less than 10 ppm (preferably less than 1 ppm), which can berestated in terms of a desired solubility-product constant of 4×10⁻¹⁰ orless. For example, CuS has a solubility-product constant of 8.5×10⁻⁴⁵(at 18° C.). Represented in terms of the solubility of CuS in water:that is, the quantity of CuS that dissolves in a liter of water, CuS issubstantially insoluble in water having an extremely low solubility ofabout 5.9×10⁻²¹ g Cu/1. The metal contents in the catalyst can be fromabout 0.1 to 30wt % and preferably from about 0.5 to 20 wt.% based onthe total catalyst weight.

The catalyst can be shaped in the form of extrudates, cylinders,multi-lobes, pellets, granules, or structure shaped (similar to thepackings of static mixers).

It has been found that Cu can be leached from the porous adsorbentsubstrate when treating ammonia-containing waste water. The preferredway to avoid this Cu leaching problem is by limiting the amount of Cuexchanged onto a zeolite substrate, such as zeolite A, zeolite X,zeolite Y, ZSM-5, erionite, or chabasite, by partially ion exchangingthe Cu onto the zeolite. The extent of the ion exchange should belimited to about 1% to about 90% (preferred about 5% to 75%) of theexchange capacity of the zeolite.

A packed bed provides an effective and efficient contactor. In thepacked bed, the reaction zone proceeds along the direction of flow. Tominimize the pressure drop across the bed and alleviate potentialplugging by debris, the reactor can be operated with the bed expanded bygreater than 5%. The reactor also can be operated at conditions for anebullient bed, a fluidizing bed, or a spouting bed. The use of filtersor guard beds may also be helpful to avoid plugging the catalyst bed.

Air, readily available, is the preferred oxidizing agent; however, otheragents include ozone and molecular oxygen, O₂. Representations for themechanisms for the various oxidation processes follow:

1. Oxidation of Sulfides

    S.sup.═ +20.sub.2 →SO.sub.4.sup.═

2. Oxidation of Sulfites

    SO.sub.3.sup.═ +0.5 O.sub.2 →SO.sub.4.sup.═

3. Oxidation of Thiosulfates

    S.sub.2 O.sub.3.sup.═ +20.sub.2 +20H.sup.- →2SO.sub.4.sup.═ +H.sub.2 O

4. Oxidation of Mercaptans

    2RSH+0.5 O.sub.2 →RSSR+H.sub.2 O

5. Oxidation of Disulfides

    S.sub.2 +40.sub.2 →2SO.sub.4.sup.═

All the reaction products are innocuous. The treated water (oxidizedeffluent) is discharged, while the gas is treated, flared, orincinerated. Any skim oil present can be recovered in an oil-waterseparator, preferably a separator drum.

The invention can be incorporated into an existing waste water treatmentsystem as shown in FIG. 1A where the waste water containing, forexample, sulfides, flowing through line 10 is mixed with air flowingthrough line 11 and the combined stream is fed to an existing typicaloxidizing tower unit 12 to convert most of the sulfides in the wastewater to thiosulfate. Any resulting gases exit unit 12 via line 13. Theoxidized water effluent containing the thiosulfate exits unit 12 vialine 14 and is then mixed with air flowing in line 15 and the combinedstream passes through the reaction zone of this invention 16, containingthe insoluble metal deposited on a porous adsorbent substrate, toconvert the thiosulfate to sulfate. The reaction conditions for thisinvention to be maintained in reaction zone 16 are as follows:

    ______________________________________                                        Process Variable                                                                              Broad Range Preferred Range                                   ______________________________________                                        Pressure, psia  10 to 1000  14.7 to 200                                       Temperature, °F                                                                        30 to 400   100 to 300                                        LHSV, v/v Hr.   0.1 to 100  1 to 20                                           O.sub.2 in Air/COD, mole/mole                                                                 1 to 100    1 to 10                                           pH              6 to 12     7.5 to 10.5                                       ______________________________________                                    

Where LHSV is liquid hourly space velocity and COD is chemical oxygendemand.

The resulting gas is separated from the liquid and the excess gas,flowing through line 17, is subsequently treated, flared, or incineratedand then discharged. Treated liquid effluent, flowing through line 19,is the product low in chemical oxygen demand and can be discharged.

In the preferred form, the invention is carried out in a fixed bedcontactor in the liquid phase which oxidizes sulfides directly tosulfates. As shown in a preferred embodiment of the process in FIG. 1B,waste water is passed through line 20 and is mixed with an oxidizingagent, preferably air, flowing through line 22 and the combined streamis fed to a fixed bed reaction zone 25, the reaction zone containing thecatalyst required for the present invention, such as, for example, Cupartially ion exchanged onto zeolite A. The waste water flows at aliquid hourly space velocity (LHSV) ranging from about 0.1 to about 100hr⁻¹, preferably from about 1 to about 20 hr⁻¹. The rate of flow of thewater is attributable to the pressure imposed on the stream by theupstream processing unit. The reaction zone is maintained at thetemperatures of the wastewater stream, ranging from about 30° F. toabout 400° F., preferably from about 100° F. to about 300° F., andpressures ranging from about 10 to about 1000 psia, with about 14.7 toabout 200 psia preferable The amount of oxidizing agent mixed with thewaste water is sufficient to provide about 1 to about 100 (preferablyabout 1 to about 10) times the stoichiometric requirement for oxidizingthe oxidizable components in the waste water which include sulfides,sulfites, thiosulfates, mercaptans, and/or disulfides (i.e. the chemicaloxygen demand of the water). After oxidation in the reaction zone, thegas is separated from the treated effluent and is discharged throughline 27 for additional treating, flaring, or incineration and thetreated water, which is low in chemical oxygen demand, is dischargedthrough line 29.

An embodiment of this invention that is suitable for replacing anexisting oxidizing unit is shown in FIG. 2. In this embodiment of theinvention, waste water containing, for example, sulfides, is fed throughline 30 to preheat exchanger 31 which heats the waste water, then ismixed with steam from line 33 and air from line 34 in a static mixer 46contained in line 32 to promote complete mixing of the air and wastewater. If required, steam from line 33, under the control of temperaturecontrol (TC) 45, is added to the waste water to increase the temperatureof the waste water stream to the level desired by the refiner. Thetemperature of the process may range between about 100° F. and about400° F., preferably about 150° F. to about 300° F. The air from line 34,under the control of flow control (FC) 35, is added to the waste waterin a sufficient quantity to provide about 1 to about 100 (preferablyabout 1 to about 10) times the stoichiometric requirement of oxygen foroxidizing the oxidizable components in the waste water which includesulfides, sulfites, thiosulfates, mercaptans, and disulfides (i.e. thechemical oxygen demand of the water).

The water and air mixture is then fed through line 32 into the oxidizingtower 36 where it contacts the catalyst and the toxins are oxidized,thus reducing the chemical oxygen demand of the waste water. In thepreferred embodiment, the catalyst is separated into three catalyst bedswith interbed distribution and mixing nozzles 37. After leaving theoxidizing tower 36 via line 38, the mixture exchanges heat with thewaste water feed in the preheat exchanger 31. The mixture then flowsfrom the preheat exchanger 31 through line 39 to a separator drum 40.

In the separator drum, the gas is separated from the treated water. Anyseparable oil contained in the waste water feed is also separated fromthe gas and treated water products. The separated gas stream flows fromthe separator drum 40 through line 41 through a pressure control station44 and to any required treating, flaring, or incineration. The skimmedoil flows from the separator drum 40 through line 42 to any neededrecovery or reprocessing. The treated water flows from separator drum 40through line 43 to discharge.

EXAMPLE

In an experimental embodiment of the invention, water insoluble Cu wasdeposited on the surface of a zeolite, Linde 13X extrudate. To make thecatalyst, CuX-7, 5 g of the 13X extrudate was ion exchanged with 100 ccof 0.05N Cu(NO₃)₂ at 50° C. for 2 hours. After this ion exchange, thecatalyst, CuX-7, contained about 3 wt.% cooper. Since the total inonexhange capacity of the 13X zeolite for copper is about 22 Wt.%, theCuX-7 was exchanged with copper to about 13.6% of its capacity. TheCuX-7 was then heated in air to 450 °C. at 1° C./min. and was kept at450° C. for 4 hours. Samples of petroleum refinery sulfide oxidationtower feed and product were obtained and analyzed. These samples werefound to have the following characteristics:

    ______________________________________                                        Sample             Thiosulfate, ppm                                                                             pH                                          ______________________________________                                        Sulfide oxidation tower feed                                                                     9.6            8.6                                         Sulfide oxidation tower product                                                                  24.6           6.5                                         ______________________________________                                    

The test procedure included packing 2 cc of the CuX-7 catalyst (20×40mesh) into a 1/4" stainless steel tubular reactor. The test water waspumped upflow using a positive displacement pump while flowing airconcurrently. The air flow rate was controlled using a mass flow meterat the lowest setting of 6.3 cc/min. The pressure was controlled atabout 5 psig. The temperatures were varied through the test. The productwas collected to analyze the thiosulfate concentration to determine theefficacy of the process at reducing the waste water thiosulfateconcentration and by extension the waste water COD. The results of thetest are shown in Table 1:

                                      TABLE 1                                     __________________________________________________________________________    Catalytic Oxidation of Sulfide Containing Water                                         Cumul.                                                                  Test  Bed Temp.                                                                             LHSV Pres.                                                                             Air S.sub.2 O.sub.3.sup.=                                                              S.sub.2 O.sub.3.sup.=                     Run Feed  Vol's                                                                             °C.                                                                        v/v Hr.                                                                            Psig                                                                              cc/cc                                                                             ppm  Red'n %                                   __________________________________________________________________________    1   Effluent.sup.(1)                                                                    --  --  --   --  --  24.6 --                                        2   Effluent                                                                             30 100 5    5   76  1.7  93                                        3   Effluent                                                                            113 100 5    5   76  1.9  92                                        4   Feed.sup.(2)                                                                        --  --  --   --  --  9.6  --                                        5   Feed  337 100 5    5   38  1.4  94.sup.(3)                                6   Feed  377 100 5    5   38  1.7  93.sup.(3)                                7   Feed  417 80  5    5   38  4.1  83.sup.(3)                                8   Feed  497 80  5    5   38  4.1  83.sup.(3)                                9   Feed  537 60  5    5   38  13.6 45.sup.(3)                                10  Feed  617 60  5    5   38  10.9 56.sup.(3)                                11  Feed  692 60  25   5   7.6 19.1 --                                        12  Feed  716 60  25   5   7.6 19.4 --                                        13  Feed  792 60  50   5   3.8 30.0 --                                        14  Feed  810 60  50   5   3.8 28.8 --                                        15  Feed  892 60  100  5   1.9 23.2 --                                        16  Feed  917 60  100  5   1.9 21.8 --                                        17  Feed  994 40  5    5   38  10.9 --                                        __________________________________________________________________________     .sup.(1) Effluent is sulfide oxidizer tower effluent.                         .sup.(2) Feed is sulfide oxidizer tower feed.                                 .sup.(3) These S.sub.2 O.sub.3.sup.=  reduction efficiencies are based        upon the oxidation intermediate, thiosulfate, at 24.6 ppm.               

I claim:
 1. A method for reducing the chemical oxygen demand of wastewater comprising the steps of:(a) feeding a cyanide-free wastewatercontaining a reducing compound selected from sulfide, thiosulfate,sulfite, mercaptan, or disulfide along with a source of oxygen into areaction zone containing a porous solid substrate comprising a materialselected from the group consisting of zeolite and activated carbonhaving a water insoluble copper compound deposited thereon; (b)contacting said waste water and said source of oxygen with saidsubstrate at a temperature of less than 300 degrees F. and a pressureless than 200 psia which catalytically oxidizes said reducing compound;and (c) discharging the waste water from the reaction zone whereby saidwaste water has a substantially lower concentration of said reducingcompound.
 2. The method as described in claim 1 in which the waterinsoluble copper compound comprises a sulfide.
 3. The method asdescribed in claim 1 in which the water insoluble copper compoundcomprises an oxide.
 4. The method as described in claim 1 in which thewater insoluble copper compound comprises CuS, or Cu₂ S.
 5. The methodas described in claim 3 in which the water insoluble copper compoundcomprises Cu₂ O or CuO.
 6. The method as described in claim 1 in whichthe source of oxygen comprises air.
 7. The method as described in claim1 in which said waste water is fed to said reaction zone at a liquidhourly space velocity of 0.1 hr⁻¹ to 100 hr⁻¹.
 8. The method asdescribed in claim 1 in which the oxygen source is fed into saidreaction zone in proportion to the feed rate of reducing compounds of 1to 100 mole O₂ /mole of reducing compound.
 9. The method as described inclaim 6 in which the air is fed into said reaction zone in proportion tothe feed rate of reducing compounds of 1 to 100 mole O₂ in air/mole ofreducing compound.
 10. The method as described in claim 1 in which theporous solid substrate comprises a zeolite selected from zeolite A,zeolite X, zeolite Y, ZSM-5, erionite, chabasite.
 11. The method asdescribed in claim 1 in which said waste water feed is heated beforeentering said reaction zone.
 12. The method as described in claim 1 inwhich said waste water feed and said source of oxygen are completelymixed before entering said reaction zone.